A light emitting diode (LED) is a semiconductor optical device capable of producing light in the infrared, visible or ultraviolet (UV) region. LEDs emitting in the visible and ultraviolet are made using gallium nitride (GaN) and its alloys with indium nitride (InN) and aluminum nitride (AlN). These devices generally consist of p and n-type semiconductor layers arranged into a p-n junction. In a standard LED device, semiconductor layers are evenly grown onto a polished substrate such as silicon. A typical semiconductor layer is composed of gallium nitride (GaN) that has been doped to be a p or n-type layer.
Important figures of merit for a LED are its internal quantum efficiency (IQE) and light extraction efficiency. For a typical LED the IQE depends on many factors, such as the concentration of point defect, Auger processes and device design. In the case of Nitride LEDs grown along polar (0001) and (000-1) directions the internal efficiency is also reduced due to the distortion of the quantum wells between the n- and p-doped layers caused by the internal electric fields. The light extraction efficiency of LEDs based on GaN is determined from Snell's law to be 4%. An LED commonly includes several quantum wells made of a small energy gap semiconductor (well) and a wider bandgap semiconductor (barrier) Visible LEDs employ indium gallium nitride (InGaN) as the well and GaN as the barrier. Ultraviolet LEDs employ AlGaN of different compositions as both wells and barriers. The internal quantum efficiency of an LED device based on nitride semiconductors grown along polar direction is reduced by electric fields across its quantum wells. This phenomenon is referred to as the quantum confined Stark effect (QCSE). The QCSE affects LED light emission by red shifting the emission wavelength and reducing photoluminescence intensity. The rather small value of light extraction efficiency is the result of the high refraction index of the semiconductor layer at the exit interface.